1. Field of the Invention
This invention relates to the field of information transmission systems, and especially to cable television distribution systems, and more particularly to converter-tuner stations which are individually addressable by means of command word signals on a cable to enable reception of selected secure channel signals carried on the cable.
2. Description of the Prior Art
Many types of tuner-converter units for subscription systems are known. Such units typically provide means for alternately directing the cable input, bearing both standard broadcast and secure channels, directly to the television set, or directing such input through the tuner-converter before presenting it to the input of the receiver. In the former state, the standard broadcast channels are receivable, the secure channels being not receivable because of the fact that their frequencies are outside the broadcast band. In the latter, selected of the secure channels are rendered receivable by conversion on one of these unused standard broadcast channels.
Such a tuner-converter system in the secure channel viewing mode, normally involves filtering the input to strip off the standard broadcast channels, presenting the secure channels to a mixer, and mixing therewith a heterodyning signal generated by a variable local oscillator. This sequence of events, depending on the oscillator frequency chosen, yields a signal which falls within one of the standard broadcast television channels, and which, when directed to the input of the receiver, can be detected.
Several significant problems exist with prior art devices of this type. Firstly, the local oscillator does not have a constant frequency, but tends to drift. This makes necessary the provision of means for fine tuning the local oscillator. Such fine tuning may not be compatible for use with the fine tuning of the television receiver itself. For example, if the local oscillator is fine tuned in an extreme position, it may not be possible to pick up the transmitted secure channel on the television set at all, reception being out of reach of the set's fine tuning adjustment.
Conventionally, the local oscillator frequencies are chosen to be significantly higher than those of the secure channel carriers. This is done in order that the fundamental or harmonic frequencies of the oscillator be maintained above the midband and high band channels, thereby avoiding interference signals which may be picked up by the receiver. The use of local oscillator frequencies above the midband frequencies requires relatively complex circuitry to develop such high frequencies.
Often, unwanted frequencies enter the mixer from the cable, passing through the relatively wide pass band of the filters which preceed it. These extraneous signals cause obvious problems relating to noise and interference in the ultimately received signal.
Fundamental or harmonic frequencies generated by the local oscillator of one station can be propagated onto the main cable system, potentially causing trouble at other subscriber stations. In the past, complex shielding and filtering has been a partial solution to this problem.
Accordingly, it is an object of this invention to provide a converter-tuner having a local oscillator which is relatively drift-free and does not require a fine tuning adjustment of its own.
It is a further object of this invention to provide a converter-tuner for a cable television distribution system incorporating more selective filtration and more precise mixing than is available in the prior art.
It is a further object of this invention to provide means for nullifying the effects of radiated frequencies from the local oscillator into other elements of the system, including the main cable.
It is a further object of this invention to provide a cable distribution television system in which the local oscillator frequencies present only minimal problems of interference and noise in other parts of the system.